1. Field of the Invention
The working of any electron tube is based on the existence, inside the tube, of an electron beam. Any electron tube should be provided with an electron injecting device. The characteristics of the electron beam given, for example its intensity and its directivity, vary according to the type of tube considered. Nevertheless, in most cases, it is necessary to have a substantially parallel, narrow and intense electron beam. These constraints are, for example, indispensable for fitting out a microwave tube, such as a travelling wave tube or a klystron or a Crookes' tube (X-ray emitter) or a cathode-ray tube.
2. Description of the Prior Art
There are prior art electron injecting devices designed to be fitted into an electron tube. A frequently used prior art device uses thermoelectronic emission, namely, the emission of electrons by certain metals (hereinafter called thermoemissive metals) when they are heated, the intensity of this emission being all the greater as the metals are hot. This prior art device generally has a heat conducting plate, one face of which is coated with a layer of a thermoemissive metal and is pointed towards that zone of the tube where the electrons have to be injected. This plate forms a cathode and is heated by means of a filament in which there flows an electrical current. An anode is placed in the tube. The current created by the electrons emitted by the heated tube increases, at a given temperature, and for a given emissive surface, with the difference in potential applied between the cathode and the anode, and then reaches a saturation value which is all the greater as the temperature is high. This current is therefore limited by the very principle of operation of the prior art device.
Another prior art device works according to the same principle, but has a different geometry: the plate is replaced by a hollow cylinder, the external surface of which is coated with a thermoemissive metal, and the filament is in the hollow of the cylinder.
These two prior art devices have two main drawbacks, in addition to the above-mentioned limits, based on their working principle, on the electron current that they can provide. Firstly, they generate a divergent electron beam (as thermoelectronic emission is a substantially isotropic phenomenon, the directions in which the electrons ejected outside the thermoemissive material are emitted are distributed in a cone corresponding to a solid angle substantially equal to 2.pi. steradian) and, consequently, need to be accompanied by devices for the concentration of the directions of the emitted electrons, thus increasing the complexity of the means for injecting electrons into the tube. Secondly, these prior art devices have a duration of use restricted by a deterioration of the thermoemissive metal layer (this layer carried to high temperature, tends to get sublimated, i.e. to go from the solid state to the gaseous state).